This invention relates to laser initiated explosive reactions for producing sintered products and, in particular, to a laser initiated reaction that produces sintered silicon dioxide (SiO.sub.2) which can be used to manufacture preforms for production of optical fibers, fiber optic waveguides and insulators for integrated circuits.
Optical waveguides having a core of a first material with a first index of refraction, n.sub.1, and a cladding of a second material with an index of refraction different than the first index of refraction, n.sub.2, are well known and effective transmitting media when n.sub.1, n.sub.2 and the diameter of the core are properly coordinated to limit transmitted light to preselected modes. In general, the index of refraction of the core will be higher than the index of refraction of the cladding. A discussion of the physical requirements of satisfactory transmitting media can be found in U.S. Pat. No. 3,659,915 issued to Maurer et al on May 2, 1972 and in references cited therein.
Optical waveguides having cores of fused silica doped with a multivalent metal oxide to increase the index of refraction are especially useful. The cladding can then be an undoped high purity fused silica which has a lower index of refraction.
Optical waveguides of this type are manufactured, for example, by the method disclosed in U.S. Pat. No. 3,933,454 issued to DeLuca on Jan. 20, 1976. As disclosed therein, at least one coating of glass (SiO.sub.2) soot or powder is deposited by a flame hydrolysis process on a starting member or seed rod. The soot coating is heated to a consolidation temperature in an atmosphere containing helium and chlorine in order to form a dense glass layer from which water has been substantially removed. After consolidation, the starting member can be removed and the resulting structure is drawn into a waveguide fiber. The DeLuca process is commonly used in the manufacture of optical waveguide fibers.
U.S. Pat. No. 4,343,687 issued on Aug. 10, 1982 to Ronn, one of the named inventors herein, discloses a method for initiating chain reactions using laser energy. At least two gaseous compounds capable of sustaining a chain reaction are introduced into a bounded space. A laser beam of sufficient fluence to initiate a chain reaction is irradiated into the space and a powder material is produced. The process can be used to deposit a powdered product onto a support such as alumina pellets, glass slides or silicon wafers.
Other laser syntheses of powder material are also known. These include, for example, synthesis of refractory oxide powders by the method disclosed in U.S. Pat. No. 4,548,798 issued to Rice on Oct. 22, 1985 wherein refractory oxide powders are obtained by pyrolyzing a metal and oxygen-containing organic chemical species that is capable of absorbing laser energy in a vibrational mode. In addition, U.S. Pat. No. 4,558,017 issued to Gupta et al on Dec. 10, 1985 relates to production of ultrafine powders such as metal silicide powder and silicon. Gaseous reactants including a metal halide and a silicon-containing compound are exposed to high intensity light, such as laser light, to yield the ultrafine powder by a photolytic reaction.
One disadvantage of producing a powder material for use in forming a glass material such as an optical waveguide fiber is that the powder material must be consolidated or sintered to form the dense glass material. Consolidation or sintering processes are generally performed using an oxy-hydrogen flame burner or other relatively expensive and impure heating method. As a result, impurities may be introduced into the final glass product, often rendering the product unsatisfactory for use as a waveguide.
It is, therefore, desirable to provide a process for producing a sintered product such as sintered silicon dioxide in a single reactive step that overcomes the disadvantages of prior art sintering processes.